Oxygen scavengers independent of transition metal catalysts

ABSTRACT

A composition for reducing the concentration of molecular oxygen present in an atmosphere or liquid, comprising at least one reducible organic compound which is reduced under predetermined conditions. The reduced form of the compound being oxidizable by molecular oxygen, wherein the reduction and/or subsequent oxidation of the organic compound occurs independent of the presence of a transition metal catalyst.

This application is a 371 of PCT/AU93/00598 filed Nov. 24, 1993.

FIELD OF THE INVENTION

This invention relates to compositions for reducing the oxygenconcentration present in an atmosphere or liquid (often referred to asoxygen scavenging). In one particular application, the compositions areused in or in association with food packaging.

BACKGROUND OF THE INVENTION

A wide variety of foods and other materials are susceptible to loss inquality during storage under atmospheric levels of oxygen. The damagecan arise from chemical oxidation of the product, from microbial growth,and from attack by vermin--much of which may be avoided by reducing theoxygen availability in the environment of the materials. In the field ofpackaging, relatively low-oxygen atmospheres have traditionally beengenerated by vacuum packing and inert gas flushing. Such methods arenot, however, generally applicable for various reasons. For example:

soft porous foods such as cakes cannot be subjected to strong vacuum;

fast filling speeds generally preclude substantial evacuation of orthorough inert gas flushing of food packages;

filling some gas-flushed containers, such as beer bottles often resultsin occlusion of air;

evacuation or flushing offers no residual capacity for removal ofoxygen, which may have desorbed from the food or entered the package byleakage or permeation.

As a consequence there has been much interest in chemical techniques forgenerating low-oxygen atmospheres and deoxygenating liquid orsemi-liquid foods. Thus, there are approaches based on the use ofoxidisable solids, for example porous sachets containing iron powder. Inanother technique, oxidisable MXD-6 Nylon is blended with polyester inthe walls of blow-moulded containers--the effectiveness of this dependson the presence of a cobalt salt catalyst, moreover the speed of oxygenremoval is limited by the oxygen permeability of the polyester. Furthermethods include sandwiching crystalline oxidisable material between thelayers of multilayer containers, and including a catalyst for thereaction of oxygen with hydrogen in a sandwich arrangement as above oras a deposit on the inner surface of the package.

Heterogeneous systems such as described above do not, however,adequately meet the general needs of the packaging industry, largelybecause they are often oxygen-sensitive prior to use or can be activatedonly under restricted conditions of, for example, temperature orhumidity. One method which might be used to provide oxygen scavenging inpackages as required, is disclosed in Rooney, M. L., Chemistry andIndustry, Mar. 20, 1982, pp. 197-198. This method involves the inclusionof a photooxidizable rubber and a photosensitising dye into a polymerfilm packaging material and then exposing it to visible light. Similarmethods are disclosed in Rooney, M. L. and Holland, R. V., Chemistry andIndustry, Dec. 1979, pp. 900-901 and Rooney, M. L., Journal of FoodScience, Vol. 47, No. 1, pp. 291-2294, 298. However, whilst thesemethods initiate oxygen scavenging upon illumination, they requireconstant illumination of the package in order to maintain the scavengingeffect. U.S. Pat. No. 5,211,875 proposes an alternative method intendedto avoid the problem of oxygen-sensitivity prior to use, involving anoxidizable organic compound (typically 1, 2-polybutadiene)and atransition metal catalyst (typically cobalt salt). Oxygen scavenging isinitiated by exposing the composition to an electron beam, orultraviolet or visible light.

SUMMARY OF THE INVENTION

However, the inclusion of a transition metal catalyst has a number ofdisadvantages including added cost, solubility difficulties, and a"gritty" appearance and reduced transparency of films made from suchcompositions. Some transition metal catalysts are also considered toxicand may not, therefore, be used with food.

The present invention avoids the disadvantages of including a transitionmetal catalyst. It may be based on plastic or other polymer-basedcompositions which can be activated as required, to effect reduction ofambient oxygen levels. The oxygen scavenging effect of the presentinvention also does not depend upon constant illumination with visiblelight.

Accordingly, in a first aspect the present invention provides a solidphase composition for reducing the concentration of ground statemolecular oxygen present in an atmosphere or liquid, comprising at leastone reducible organic compound which is reduced when the composition issubjected to predetermined conditions, the reduced form of the compoundbeing oxidizable by ground state molecular oxygen, wherein the reductionand/or subsequent oxidation of the organic compound occurs independentof both constant illumination with visible light and the presence of atransition metal catalyst.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, the reduction and/or subsequent oxidation of the at leastone reducible organic component is also independent of the presence ofan alkali or acid catalyst.

The reducible organic compound for use in this invention may be reducedunder predetermined conditions such as by exposure to light of a certainintensity or wavelength or, alternatively, by the application of heat,γ-irradiation, corona discharge or an electron beam. Possibly, also, thecompound may be reduced by incorporating in the composition a reducingagent which in turn can be activated under predetermined conditions, sayby heating.

Typically the reducible organic compound will be a compound having thecapacity to be converted to an excited state such as a triplet form,which then becomes reduced to an essentially stable state by gaining orabstracting an electron or hydrogen atom from other molecules or byredistributing an electron or hydrogen atom within the compound itself.The reduced molecule is reactive towards molecular oxygen to produceactivated species such as hydrogen peroxide, hydroperoxy radical or asuperoxide ion. Preferably, the reducible organic compound is stable inair at room temperature or is in its fully oxidized state. Examples ofsuitable compounds include quinones, such as benzoquinone, anthraquinone(preferably, 9,10-anthraquinone) and naphthoquinone (preferably,1,4-napthoquinone); and photoreducible dyes and carbonyl compounds whichhave absorbance in the UV spectrum, such as azo, thiazine, indigoid andtriarylmethane compounds.

Most preferably, the reducible organic compound is a substitutedanthraquinone such as 2-methylanthraquinone and 2-ethylanthraquinone. Insome applications, 2-ethylanthraquinone shall be preferred to2-methylanthraquinone due to its greater solubility.

The reducible organic compound component may comprise 0.1-99.9 wt % ofthe composition. More preferably, the reducible organic compoundcomprises 0.1-50 wt % of the composition.

Compositions of this invention which involve the formation of anactivated oxygen species (eg, peroxide) may further comprise ascavenging component reactive towards the activated species. This may beembodied in the reducible organic compound itself, for example a quinonehaving an amine group would be effective, but in any event it should bean agent which is substantially stable in contact with air at roomtemperature. Suitable examples of the activated oxygen scavengingcomponent include antioxidants such as alkylated phenols and bisphenols,alkylidene bis-, tris- and polyphenols, thio- and bis-, tris- andpolyalkylated phenols, phenol condensation products, amines,sulfur-containing esters, organic phosphines, organic phosphites,organic phosphates, hydroquinone and substituted hydroquinones;inorganic compounds such as sulphates, sulfites, phosphites and nitritesof metals, particularly those of groups 1 and 2 of the periodic tableand first row transition metals, zinc and tin; sulfur-containingcompounds such as thiodipropionic acid and its esters and salts,thio-bis(ethylene glycol β-aminocrotonate), as well as the amino acidscysteine, cystine and methionine; and nitrogen-containing compoundscapable of reacting with activated forms of oxygen include primary,secondary and tertiary amines and their derivatives including polymers.

Preferably, the scavenging component reactive towards the activatedoxygen species is selected from the group consisting oftriphenylphosphine, triethylphosphite, triisoproppylphosphite,triphenylphosphite, tris(nonylphenyl) phosphite, tris(mixed mono- andbis-nonylphenyl) phosphite, butylated hydroxytoluene, butylatedhydroxyanisole, tris(2,4-di-tert-butylphenyl) phosphite,dilaurylthiodiprpionate, 2,2-methylene-bis-(6-t-butyl-p-cresol),tetrakis(2,4-d-tert-butylphenyl)4,4'-biphenylene diphosphonite,poly(4-vinylpyridine) and mixtures thereof.

The activated oxygen species-scavenging component may be in the form ofa polymer or oligomer. Such forms may be prepared by, for example,covalently bonding a compound such as those activated oxygenspecies-scavenging compounds listed above to a monomer or co-monomer. Alimitation on the molecular size of the activated oxygenspecies-scavenging component will be the effect, if any, it has onfunctional properties of any other polymer with which it is combined asin blending for instance.

The activated oxygen species-scavenging component may comprise 0.1 to99.9 wt %, more preferably, 0.1 to 50 wt % of the composition.

As an alternative to components which can be excited to a state whichconverts oxygen to an activated species, compositions according to thisinvention may comprise components which are excitable to a state inwhich they react and bind directly with oxygen diffusing into thecomposition from the surroundings.

The compositions according to the invention may further comprise anadhesive (eg, a polyurethane such as LAMAL) and/or a polymer. Preferredpolymers are homogenous and include polyvinyls, polyolefins andpolyesters or their copolymers, ethyl cellulose and cellulose acetate.Heterogeneous substrates, eg inorganic polymers such as silica gel orpolymer mixtures may also be used.

Alternatively or additionally, the reducible organic compound itselfmaybe in a polymerised form either as homopolymers or copolymers.Oligomer forms may also be suitable. Reducible monomers can be made bycovalently bonding an ethylenically unsaturated group to a reducibleorganic compound. The reducible organic compound may also carry groupscapable of reaction with other polymerisable molecules and preformedpolymers. Particular examples of ethylenically unsaturated reduciblemonomers include vinyl and isopropenyl derivatives, preferably bonded tothe reducible organic compound in such a manner as not to decrease thelifetime of the triplet excited state compared with that of theunsubstituted reducible organic compound. Thus in the case of9,10-anthraquinone substitution occurs preferably at the 2, 3, 6 or 7positions. If such a reducible organic compound carries additionallyfurther substituents besides the vinyl or isopropenyl group, suchsubstituent should preferably be in one or more of the remainingpreferred positions.

Co-monomers can be any ethylenically unsaturated substance whethermono-unsaturated, di-unsaturated or polyunsaturated. Examples includealkenes of carbon number two to eight, vinyl acetate, vinyl alcohol,acrylic monomers including methacrylic and acrylic acids, their amides,esters and metal salts as in ionomers, acrylonitrile, methacrylonitrile,norbornene, norbornadiene. If the reducible organic compound is asubstituted 9,10-anthraquinone and is required to be difunctionalmonomer for formation of a polyester, the two carboxyl or hydroxylsubstituents, or their derivatives should preferably be in any two ofthe positions, 2, 3, 6 or 7.

Reducible monomers may be polymerised as condensation polymers such aspolyesters, including polycarbonates, polyamides, polyimides. An exampleof a polyamide is the polymer of 2,6-anthra-9,10-quinone dicarboxylicacid with 1,6-diaminohexane. Reducible monomers may also be polymerisedwith diisocyanates or diols to form polyurethanes or may be bonded topolyurethanes. An example of the latter is the reaction product of2-bromomethyl-9,10-anthraquinone with the polyurethane fromtoluenediisocyanate and 1,6-hexandiol.

Preferably, the composition according to the invention comprises areducible organic compound and an activated oxygen species-scavengingform, both of which are present in polymerised form(s).

Where the reducible organic compound is dispersed or dissolved in apolymer which does not readily donate a hydrogen atom or electron to thereducible organic compound in its excited state, an additional source oflabile hydrogen or electron is preferred. Such a compound is preferablyone containing a hydrogen bonded to nitrogen, sulfur, phosphorus oroxygen especially where a hydrogen is bonded to a carbon atom bonded tothe abovementioned heteroatom. Alternative sources of electrons aresalts of organic compounds such as the salts of sulfonic acids orcarboxylic acids. In one form of the invention the sodium sulfonate saltof a polymerised 9,10-anthraquinone would be used. Thus the reducibleorganic compund itself can be the source of its own electron for thereduction process.

The reduced form of the organic compound used in the composition, bringsabout a reduction in the molecular oxygen concentration in theatmosphere or liquid through its oxidation by the molecular oxygen, thereduction and/or oxidation being independent of the presence of atransition metal catalyst and, preferably, also independent of thepresence of an alkali or acid catalyst. Nevertheless, transition metalcompounds, alkaline and/or acidic agents may also be included in thecompositions where they may effect the rate of oxygen scavenging or mayenhance the reduction and/or subsequent oxidation of the organiccompound. For example, ascorbic acid may be included in the compositionscomprising anthraquinones as a photoreduction enhancer.

Reduction of the reducible organic compound may take place only whenconvenient. This might be, for example, when the composition is beingmade into or brought into association with packaging material or,alternatively and perhaps more usually, after formation of a package andprior to filling and sealing. Reduction may even be deferred until aftersealing of the package.

Thus, in a second aspect, the invention provides a method for reducingthe concentration of molecular oxygen present in an atmosphere orliquid, comprising exposing the atmosphere or liquid to a compositionaccording to the first aspect and thereafter, reducing the reducibleorganic compound.

Alternatively, the invention provides a method for reducing theconcentration of molecular oxygen present in an atmosphere or liquid,comprising exposing the atmosphere or liquid to a pre-reduced form of acomposition according to the first aspect.

The compositions according to the invention may be used independently oras components of blends. They may take the form of a cross-linkedpolymeric matrix, as in a can lacquer, or be bonded to or absorbed ontoan inorganic polymer, such as silica. They may be effectively appliedas, or incorporated in, for example, bottle closure liners, PET bottles,liners for wine casks, inks, coatings, adhesives, films or sheets eitheralone or as laminations or co-extrusions, or they may take the form ofpads, spots, patches, sachets, cards, powders or granules which may beattached to packaging material or located independently within apackage.

Films comprising the composition according to the invention may bemonolayer or multilayer laminate, and may be used on their own or may beaffixed or applied to a solid substrate (eg, a solid packagingmaterial). Where the film is a multilayer film, it is preferable that anouter layer is an oxygen barrier film, so that the film may be used in amanner such that only the layer(s) containing the reducible organiccompound is exposed to molecular oxygen from the atmosphere or liquidfor which a reduction in molecular oxygen concentration is required.

Films comprising the composition may also be used is as a chemicalbarrier to oxygen transmission through a packaging material. Thus if apackaging material has a certain oxygen permeability, the oxygen passingthrough it from the outside environment into a reduced oxygen contentatmosphere within the package can be scavenged by the reducible organiccompound. The composition can be dissolved or dispersed within thepackaging material or can be placed adjacent to it as an additionallayer on the inner side of the package.

In multilayer laminate films, an activated oxygen species-scavengingcomponent may be provided in a seperate layer from the layer comprisingthe activatable component.

Film layers containing a reducible organic compound may be formed eitherfrom molten plastic compositions extruded to give a particular shape ordimensions or from a liquid state which gives the final solid layer byreaction, or evaporation of a volatile liquid. Plastic compositions willoften be extruded at temperatures between 50° C. and 350° C. dependingupon chemical composition and molecular weight distribution. Extrusionmay be via a die to give a film layer either alone or as a componentlayer of a multilayer coextrusion. The layer comprising the reducibleorganic compound may be extruded onto another substrate as in extrusioncoating and lamination. Extrusion may be followed by moulding as ininjection or blow moulding. These processes can involve the formation offoams in some instances.

The composition according to the invention may also take the form of aprinting ink, coating or lacquer. These may or may not be pigmented. Theprinting inks, coatings and lacquers will normally be applied in aliquid state and solidified by evaporation of the solvent or dispersionmedium or by reaction of some of the constituents.

While the composition and methods according to the invention are likelyto be of particular value in food-packaging situations where oxygenremoval is desirable, their utility is not limited thereto. Otherapplications include, for example, the generation of low-oxygenatmospheres in vessels for anaerobic or microaerophilic microbiology, orthe generation of low-oxygen gas for blanketing flammable oroxygen-sensitive materials. The technology can also be used inconjunction with technologies based on other means of oxygen scavengingsuch as photosensitized generation of carbon dioxide.

Compositions according to the invention may be re-reduced, if necessary,by resubjecting to the predetermined condition to recommence oxygenscavenging. This may be particularly useful if the composition has beenexposed to air prior to package sealing. Re-reduction may be achieved ata light intensity as low as ambient room illumination for an hourdepending upon the amount of re-reduction required.

In addition to the advantages disclosed above, the method can, in someinstances, be practised with compositions formulated to beself-indicating in respect to their capacity for oxygen removal. Thatis, some reducible organic compounds upon reduction will undergo achange in colour or change in UV-visible, infrared or near-infraredabsorption spectrum. For example, photoreduction of quinones and some oftheir derivatives results in a spectral shift from the UV to longerwavelengths, especially to the visible region of the spectrum; byincorporation of such compounds, package material can be formulatedwhich will undergo a colour change as the capacity for reducing theoxygen concentration becomes exhausted.

This colour change also provides a mechanism for checking whether all ofthe reducible organic compound in the composition has been reduced.Where reduction is found to be incomplete, the composition may beresubjected to the predetermined conditions. Further, such compositionsmay also be used as an indicator of seal breakage. That is, in the areaof film where a heat seal or other seal is made between the filmcontaining the reducible organic compound and a material of sufficientlyhigh oxygen barrier, oxygen cannot reach the reducible organic compoundas fast as it can in other areas. The seal area therefore remainscoloured due to the presence of reduced organic compound. Thefluorescent emission from the reduced organic compound is particularlyuseful for this purpose. A green fluorescence is seen when9,10-anthraquinone with substitutents in the 2-position are bonded by amethylene group to the ring. Alternatively, a strip or ring of thecomposition may be located on the inner side of the sealed packageadjacent to the seal. Where the seal is formed by an adhesive, thecomposition may comprise the adhesive. If the seal should be incompleteor become broken in any way, then this may be detected by a colourchange in the composition.

Visible colour changes may be detected by eye, whereas changes inUV-visible, infrared or near infrared absorption spectrum may bemeasured with an appropriate device such as a photocell used with alight source of appropriate wavelength and intensity.

The invention will now be further described with reference to thefollowing non-limiting examples and figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Photoreduction of 2-methyl-9,10-anthraquinone (0.075M in ethylcellulose, heat sealed between two layers of PVDC-coated polypropylene).Spectra show the photoreduction of the sample under xenon lampirradiation (0, 30, 90, 180, 300 and 600 seconds) and detail the declineof the peak at 327 nm, with a corresponding increase in the peak at 375nm, as irradiation time increases.

FIG. 2: Photoreduction of 2-methyl-9,10-anthraquinone (0.075M in ethylcellulose, heat sealed between EVOH barrier laminate and SURLYN1601)under xenon lamp irradiation (0, 30, 90, 180, 300 and 600 seconds). Thepeak at 327 nm decreased with increasing irradiation time, while thepeak at 375 nm increased until approx. 300 s irradiation, when it beganto decrease.

FIG. 3: Reoxidation of reduced species in air after storage for 106 daysunder nitrogen. The reoxidation involves the gradual loss of the peak at375 nm with a corresponding increase in the peak at 327 nm. The threespectra shown above correspond to the film sample reduced, and thenafter 1 day and 21 days left to oxidise.

EXAMPLE 1

Photoreduction and reoxidation of anthraquinone in ethyl cellulose.

9,10-Anthraquinone, 0.05 g, and ethyl cellulose (degree of substitution,2.5), 1.5 g, were dissolved in ethyl acetate, 12 ml. The solution wascast onto plastic coated glass to give a film 10-15μ thick when thesolvent had evaporated. A sample of the film was placed in a nitrogenflushed spectrophotometric cuvette and the absorbance at 252. nm and264. nm was measured.

While still in the cuvette under nitrogen, the sample was irradiated for20 seconds with light from a xenon lamp using a Suntest CPS instrument,and the ratio of absorbance at 254.8 to that at 264.3 measured. Thesample was exposed to air for a total of 10 days to allow reoxidation ofthe anthrahydroquinone and/or the semi-quinone, which would result inthe absorbance ratio returning towards its initial value. The results inTable 1 show that photoreduction occurs on irradiation, followed byreoxidation upon exposure to air. Slight shifts in A_(max) occurred withreduction and oxidation.

                  TABLE 1    ______________________________________             Absorbance Ratio    Treatment  254.3/264.3 252.4/264.3                                     252.8/264.3    ______________________________________    None       2.10    Irradiated (20s)       0.93    3 days in air                    1.44    6 days in air                    1.49    10 days in air                   1.50    ______________________________________

EXAMPLE 2

Photoreduction of 2-methylanthraquinone in ethyl cellulose sandwichedbetween two layers of polyvinylidine chloride copolymer-coatedpolypropylene

2-methyl-9,10-anthraquinone (hereinafter referred to2-methylanthraquinone or MeAq), 0.0167 g, was dissolved in ethylacetate, 9 ml, together with ethyl cellulose, 1.15 g, and a film wascast as in Example 1. The film was 20-30 μm thick. A strip of this filmwas placed between two layers of the PVDC-coated polypropylene, and thetwo outer layers were heat sealed together to form a flat packagecontaining the ethyl cellulose layer with essentially no headspace.

The 3-layer sample was held in a steel clamp and exposed for seven timeintervals to the light from the xenon lamp as in Example 1. The maximumexposure was 10 minutes and the absorption spectrum was measured with aspectrophotometer after each exposure. The results in FIG. 1 show thedecrease in absorbance due to 2-methylanthraquinone at 327 nm withincrease in absorbance at 375 nm (and at longer wavelengths) due toprogressive formation of the corresponding anthrahydroquinone and/orsemiquinone.

These results indicate that a barrier plastic film permits thephotoreduction of the 2-methylanthraquinone without the interference byatmospheric oxygen.

EXAMPLE 3

Photoreduction of 2-methylanthraquinone in ethyl cellulose sandwichedbetween a layer of Surlyn 1601 (Du Pont, Wilmington. USA) and aco-extruded film of Surlyn. ethylene vinyl alcohol copolymer and nylon 6

A sample of the ethyl cellulose film described above in Example 2 wasplaced between one high-permeability layer (Surlyn 1601) and onelow-permeability layer (the co-extrusion) containing ethylene vinylalcohol as barrier. This would allow testing whether the low barrier issufficient to stop oxygen interference with the photoreduction.

The film sandwich was illuminated and the spectra were measured as inExample 2. The spectra in FIG. 2 show that the quinone is photoreducedbut with some decrease in efficiency compared with the previous Exampleand with some evidence of side reaction. This result indicates that theoxygen permeability of either the layer provided by the Surlyn or thelayer containing the quinone should preferably be reduced.

EXAMPLE 4

Scavenging oxygen from air in pouches made from a high-oxygen-barrierplastic

2-Methylanthraquinone, 0.05 g, was dissolved in ethyl acetate, 9 ml,together with ethyl cellulose, 1.25 g and a film cast 19 cm×18 cm on theSurlyn side of the co-extruded film described in Example 3. This samplewas termed the "control" and was folded to form a pouch of dimensions 23cm×20 cm. The pouch was flushed with nitrogen and exposed to xenon lampirradiation as per Example 3 for 3 minutes. Some nitrogen was thenremoved by syringe via a septum and air, 20 ml, was added and the oxygencontent measured by gas chromatography. (The quantity of2-methylanthraquinone is approximately equimolar with 4 ml of oxygen.)The pouch was then stored in darkness.

Three additional pouches were prepared in a similar fashion but with theaddition of triphenylphosphine to scavenge the hydrogen peroxide orformed derivative. The quantities of triphenylphosphine were 0.059 g,0.118 g and 0.295 g. These pouches were treated in the same manner asthe pouch from the "control".

The oxygen content of the hour pouches was determined again afterstorage for 25.3 hours and the initial and final results are shown inTable 2.

The film has an oxygen transmission rate of 6 cc/m² /day/atmosphere at20° C. 75% RH.

                  TABLE 2    ______________________________________    Oxygen scavenging in the presence of triphenylphosphine                 Initial   Oxygen    Final    Triphenylphosphine                 oxygen    After     oxygen    (g)          (%)       25.3 hrs (%)                                     (%)    ______________________________________    0            20.3      16.1      14.05    0.059        10.45     3.9       2.8    0.118        15        1.17      0.35    0.295        14.6      0.07      0.16    ______________________________________

Preparation of the films was repeated with an additional one containingtriphenylphosphine at a level of 0.59 g, as well as the2-methylanthraquinone, but this time the pouches were opened afterirradiation and the ethyl cellulose layers removed, placed in Quickfittest tubes and stoppered. The test tubes were stored in darkness for 24hours.

Potassium iodide, 1% w/w in water was prepared and 4 ml of this solutionwas added to each of the test tubes. Freshly prepared starch mucilage, 2ml, was also added and the test tubes were shaken vigorously for 30seconds. The test tubes were allowed to stand in darkness. After 5minutes the test tube containing the "control" film contained blackstained film due to the release of iodine formed through action of thehydrogen peroxide or its derivative. The film containingtriphenylphosphine, 0.059 g, showed a slight blue stain only after 3days. The remaining films showed no evidence of iodine formationindicating no release of hydrogen peroxide from the films.

EXAMPLE 5

Oxygen scavenging using a polymerised reducible organic compound

2-Vinylanthraquinone, 0.25 g, was dissolved in benzene, 30 ml, andbenzoyl peroxide, 0.01 g, was added. The solution was degassed byfree-thaw evacuation and polymerisation was carried out at refluxtemperature for three hours during which time a precipitate was formed.The solvent was removed and the polyvinylanthraquinone was mixed withtriphenylphosphine, 0.059 g, and octanol-1-ol, 5 drops, in chloroform.The resulting solution was cast on the Surlyn side of the co-extrusiondescribed in Example 3, with an area of approximately 25 cm×20 cm. Apouch was formed by heat-sealing this film to another piece ofco-extrusion and the air was removed by evacuation. Nitrogen wasinjected into the pouch and this was irradiated as in Example 4. Most ofthe nitrogen was removed and replaced with air, 20 ml, as in Example 4.

The oxygen content of the pouch was found to be 17.4%. The pouch washeld in darkness and the oxygen content was found to be 10.7% after 4hours and 10.6% after 22.5 hours.

EXAMPLE 6

Oxygen scavenging by 2-methylanthraquinone in a UV-cured varnish

A mixture of a commercial UV-curable varnish, 50 parts, and ethanol, 50parts, was used to dissolve 2-methylanthraquinone as a 5% solution. Thismixture was applied to a polypropylene film as a 2 to 3 μm thick layerusing a pilot scale coating machine. Strips of the coated polypropyleneapproximately 20 cm×20 cm, were cut and placed in pouches made from thefilm described in Example 4. After nitrogen flushing the pouches wereexposed to xenon lamp irradiation for periods of time shown in Table 3.The nitrogen was removed and 200-250 ml of nitrogen containing 0.5%oxygen was injected into each pouch. The scavenging of oxygen wasdetermined by gas chromatography and the volume of oxygen consumed after1 hour is shown in Table

                  TABLE 3    ______________________________________    Oxygen scavenging by a UV-cured varnish    containing 2-methylanthraquinone                   % of total oxygen scavenged    Irradiation Time (sec)                   after 1 hour    ______________________________________    1              50    2              28    3               3    4              31    5              27    6              19    ______________________________________

Similar results were obtained when ethyl cellulose was used instead ofthe UV-curable varnish.

EXAMPLE 7

Oxygen scavenging in the presence of carbon dioxide

Two pouches were prepared as described in Example 4 withtriphenylphosphine and 2-methylanthraquinone contents of 0.118 g and0.055 g respectively in the ethyl cellulose layer, 1.25 g. Afternitrogen flushing and irradiation as per Example 4 the pouches wereevacuated and filled with 20 ml each of air and carbon dioxide. Thepouches were stored in darkness and the oxygen concentration wasmonitored by gas chromatography.

The results are shown in Table 4 and comparison with the result for thecorresponding pouch without carbon dioxide after 25.3 hours storage(Table 2) shows very little difference in scavenging rate. Currentscavengers based on iron powder are often inactivated or have theirscavenging rate severely retarded by the presence of carbon dioxide.

                  TABLE 4    ______________________________________                Oxygen Concentration (%)    Time(h)       Pouch 1   Pouch 2    ______________________________________    0             8.3       7.8    5.5           6.2       6.3    22.0          3.3       3.4    46.0          1.9       1.4    ______________________________________

EXAMPLE 8

Stability of Scavenging Capability

The reactivity towards oxygen of the photoreduced 2-methylanthraquinonein ethyl cellulose after 106 days storage in the absence of oxygen wasdemonstrated as follows. Ethyl cellulose, 1 g and 2-methylanthraquinone,0.018 g, were dissolved in ethyl acetate, 9 ml, and cast as five filmsmeasuring approximately 10 cm×10 cm×200 μm on the surface of theco-extruded barrier film based on ethylene vinyl alcohol described inExample 3.

One of these coated films was made into a pouch and the air was removedand replaced by nitrogen. The film was irradiated as in Example 4 andthe pouch was stored in an atmosphere of nitrogen for 106 days. TheUV-visible spectrum was measured after which the pouch was opened toallow air to replace the nitrogen in the pouch. The spectrum wasmeasured after one day and 3 weeks.

The spectra are shown in FIG. 3 which shows that the reduced speciesreoxidises with a decrease in absorbance at wavelengths above 350 nm andand increase around 330 nm characteristic of the reoxidation of theanthrahydroquinone or semiquinone species to the quinone form.

EXAMPLE 9

Photoreduction and reoxidation of 2-methylanthraquinone in the presenceof poly(4-vinylpyridine)

A solution of 2-methylanthraquinone and ethyl cellulose in ethyl acetatewas prepared and cast into 2 separate films as in Example 4. One of thefilms was coated over part of its area with a film ofpoly(4-vinylpyridine), PVP, made by casting a solution of 0.7 g ofpolymer from methanol solution followed by solvent evaporation.

The film samples were formed into pouches and irradiated using thetechniques described in Example 4. After irradiation the pouches wereopened and the excess co-extruded barrier packaging material was cutoff. The samples were placed with the ethyl cellulose-coated sideuppermost in plastic dishes where they were left in air for 4 days.

The samples were then covered with the starch/potassium iodide solutiondescribed in Example 4. The solution covering the sample not coated withPVP turned dark blue/black within a few minutes. The solution coveringthe PVP-coated area of the second sample did not become coloured evenafter several hours whereas the solution covering the uncoated areascoloured the same as was found with the uncoated sample.

The results indicate that the PVP scavenged the oxidizing species suchas hydrogen peroxide formed in reoxidation of the photoreduced2-methylanthraquinone.

EXAMPLE 10

Oxygen scavenging in the cold

A pouch was made from film comprising 2-methylanthraquinone, 0.055 g,with ethyl cellulose, 1.25 g and triphenylphosphine 0.118 g as describedin Example 4. 20 ml of air was added and the pouch stored at -1.0 to1.0° C.

    ______________________________________    Results           Time (days)                   % oxygen    ______________________________________           0.0     19.6           1.      8.04           2.      6.22           3.      4.49           6.      3.03           13.     1.64           17.     1.21           41.     1.13    ______________________________________

EXAMPLE 11

Dependence of scavenging upon irradiation time and delay betweenirradiation and exposure to air

Ethyl cellulose, 1.2 g, and 2-methylanthraquinone, 0.118 g, weredissolved in ethyl acetate, 9 ml, and cast as four films onto the Surlynside of CSDE film and made into pouches with the test film folded overonto itself and a half slice of tissue inserted between. All were vacuumpacked in the Turbovac, then two were given a total of five minutesirradiation (half each side) and the other two ten minutes. One each ofthe pouches immediately had 20 ml of air injected, the other two beingleft overnight before they too had 20 ml of air introduced. This testedfor whether leaving the films after reduction has any effect on theiroxygen scavenging.

    ______________________________________    Results             Filled immediately                               Left overnight    Time (h)   5M      10M         5M    10M    ______________________________________    0.0        21.     21.         21.   21.    2.         6.49    5.91    4.5                            6.35  6.32    19.5       0.57    0.3    21.8                           0.75  0.29    24.        0.23    0.28    28.8                           0.27    50.        0.2    ______________________________________

The results show that irradiation time may have some effect on rate ofscavenging, but it is small between 5 and 10 minutes. Also, leaving thepouches prior to filling seems to have no effect.

EXAMPLE 12

Peroxide scavenging from a separate film

    ______________________________________    Films Cast    MeAq (g)  NQ (g)      TPP (g) EthCell (g)    ______________________________________    0.055     --          --      1.2    --        0.039       --      1.2    --        --          0.118   1.2    ______________________________________

The 2-methylanthraquinone and 1,4-naphthoquinone (NQ) films were castdirectly onto CSDE, with the triphenylphosphine (TPP) films cast ontopolethylene cling wrap, then placed on top of the others, flattened out,and the edges taped down. Holes were punched through the TPP film sothat it would not blow up when placed in the Turbovac. Bags were vacuumpackaged and given 10 minutes irradiation before having 20 ml of airinjected.

    ______________________________________    Results                   % oxygen    Time (h)         MeAq    NQ    ______________________________________    0.0              21.     21.    2.               14.45    4.8                      17.75    18.8             5.03    23.3                     12.    25.8             4.47    44.3             1.89    89.                      2.86    114.8            0.61    166.3                    1.28    187.3            0.81    ______________________________________

EXAMPLE 13

Photoreduction on the coater-laminator

    ______________________________________    Films cast    MeAq (g)     TPP (g) EthCell (g)    ______________________________________    0.055        0.118   1.2    --           0.12    1.2 (onto polyethylene                         cling film)    0.055        --      1.2    ______________________________________

Pouches were set up in a similar way to previous examples, butirradiation was with a coater-laminator at a web speed of 5 m/min. TheUV lamps cast a beam of light approximately 10 cm long, and thus thesamples were irradiated for an average of 1.2 seconds. As in previoustests, 20 ml of air was injected.

    ______________________________________    Results             % oxygen    Time (h)   Type a  Type b     Type c                                        Type d    ______________________________________    0.0        21.     21.        21.   21.    2.                            13.47 17.67    4.5                           12.91 16.12    5.3        5.95    2.05    7.         2.35    0.36       11.53 15.77    24.        0.17    77.                           2.18  10.89    ______________________________________     (a) and (b)MeAq and TPP in the one film.     (c) and (d)TPP was in ethyl cellulose cast on polyethylene cling film.

The results appear to show that the scavenging performance is unaffectedby the different method of photoreduction, and this was supported by theintense fluorescent colour of the film straight off the coater.

EXAMPLE 14

Stoichiometry of the MeAO reoxidation

    ______________________________________    Films cast    MeAq (g)       TPP (g) EthCell (g)    ______________________________________    0.055          0.118   1.2    ______________________________________

Film was set up in a similar manner to previous examples, and irradiatedfor 10 minutes with the Xenon lamp before injecting 60 ml of air (ie,oxygen:MeAq ratio of 3:1).

    ______________________________________    Results           Time (h)                  % oxygen    ______________________________________           0.0    21.           89.5   9.52           115.   9.36    ______________________________________

This result indicates a scavenging ratio of 1.7 moles of oxygenscavenged per mole of MeAq. Extraction and HPLC/V-VIS revealed that noTPP was present (2:1 molar ratio with MeAq) and this may be confirmationthat more than a 1:1 molar amount of peroxide was produced by the MeAqreoxidation.

EXAMPLE 15

Ferrous sulphate as a peroxide scavenger

    ______________________________________    Films cast    MeAq (g)      F.Sul (g)                           EthCell (g)    ______________________________________    0.055         0.344    1.2    ______________________________________

Pouches were prepared in a similar manner to the previous examples withthe ferrous sulphate heptahydrate F.Sul) (ground into fine powder)dispersed through it. The pouch vacuum packed, and irradiated for 10minutes in the Xenon lamp, before 20 ml of air was injected.

    ______________________________________            0.0  21.            2.3  16.27            18.8 7.42            25.5 5.46            114.8                 2.53            142.8                 2.07            190.8                 1.6            260.8                 3.17    ______________________________________

The results suggest that oxygen scavenging is slower than when TPP isused

EXAMPLE 16

Anthraquinone-2-aldehyde (AQ2A): inbuilt peroxide scavenging

    ______________________________________    Films cast    AQ2A (g)       TPP (g) EthCell (g)    ______________________________________    0.058          --      1.2    0.058          0.118   1.2    ______________________________________

Two films each were cast with the above quantities and vacuum packedbefore being irradiated for 10 minutes with the Xenon lamp and injectedwith 20 ml of air.

    ______________________________________    Results    Time (h)  AQ2A1        Time (h) +TPP1    ______________________________________    0.0       21.          0.0      21.    2.3       17.02        2.       8.61    19.3      15.          4.       5.33    44.8      14.52        75.5     0.62    121.5     12.44    ______________________________________

EXAMPLE 17

PEF as peroxide scavenger

    ______________________________________    Films cast    MeAq (g)       PEF (g) EthCell (g)    ______________________________________    0.055          0.072   1.2    ______________________________________

Two pouches were made, the bis(furfurylidene) penta-erythritol (PEF)being 1:1 w.r.t. MeAq, vacuum packed, irradiated for 10 minutes with theXenon lamp, and injected with 20 ml of air.

    ______________________________________    Results    Time (h)         MP1    MP2    ______________________________________    0.0              21.    21.    4.               15.91  15.32    23.3             10.84  12.94    69.              7.38   12.78    148.5            2.29   1.82    ______________________________________

EXAMPLE 18

Cellulose acetate as scavenging medium

    ______________________________________    Films cast    MeAq (g)       TPP (g) Cel.Ac. (g)    ______________________________________    0.1            0.1     1.4    ______________________________________

The pouches were prepared in a similar manner to previous examples andwere irradiated with the Xenon lamp.

    ______________________________________    Results    Time (h)         CA1         CA2    ______________________________________    0.0              21.         21.    5.3              19.2        8.57    Time (h)         CA1         CA2    23.5             1.3         1.05    30.3             5.14        0.38    ______________________________________

EXAMPLE 19

LAMAL Adhesive--TPP peroxide scavenging

    ______________________________________    Films cast    EtAq      L-HSA   L-C         EtOH TPP    (g)       (g)     (g)         (g)  (g)    ______________________________________    0.1       1.8     0.2         3.   0.24    ______________________________________     EtAq = 2ethyl-9,10-anthraguinone     LHSA = polyurethane base polymer     LC = cross linking agent for the polyurethane

Films were cast onto warm plate (covered with polyester/polyethylenelaminate) using the TLC spreader with a gap of 300μ. The adhesive wasthen laminated with PVC cling-wrap, and injected with 20 ml of air.

    ______________________________________    Results            Time (h)                   Lamal 1    ______________________________________            0.0    21.            1.5    12.01            18.5   0.62            22.3   0.47    ______________________________________

The results indicate that the films work well, although perhaps a littleslower than TPP in ethyl cellulose.

EXAMPLE 20

LAMAL--Triisoproyl phosphite (TIP) peroxide scavenging

    ______________________________________    Films cast    EtAq      L-HSA   L-C         EtOH TIP    (g)       (g)     (g)         (g)  (g)    ______________________________________    0.1       1.8     0.2         3.   0.24    ______________________________________

Two samples were prepared as in Example 19.

    ______________________________________    Results    Time (h)        Lamal2  Lamal3    ______________________________________    0.0             21.     21.    3.              13.86   17.42    21.5            9.63    7.23    45.             4.19    5.36    189.            3.61    4.26    ______________________________________

EXAMPLE 21

Triisopropyl phosphite peroxide scavenging from ethyl cellulose

    ______________________________________    Films Cast    EtAq (g)        TIP (g)             EthCel (g)    ______________________________________    0.1             0.1     1.2    ______________________________________

Film was cast onto EVOH barrier material using the TLC spreader with agap of approximately 400μ vacuum packed and irradiated as in previousexamples.

    ______________________________________    Results    Time (h)         TIP1   TIP2    ______________________________________    0.0              21.    21.    2.3              19.46  12.01    22.2             7.14   5.58    96.3             4.03   0.36    452.             0.24   0.18    ______________________________________

EXAMPLE 22

Use of gamma-irradiation for activation of ethyl cellulose films

A cobalt-60 source was used to provide a dose of 25 kilogray to filmscontaining ethylanthraquinone and triphenylphosphite. The films weremade as described below and the results provided in Table 5.

Ethylanthraquinone, 0.13 g, triphenylphosphite, 0.385 g, and ethylcellulose, 3.3 g, were dissolved in ethyl acetate and the resultingsolution was spread on two sheets of poly(ethylene-terephthalate); 12 μmthick with the aid of a doctor blade. The solvent was evaporated bywarming to approximately 40° C. for 10 minutes in a fume hood. Theresulting plastic films had an area of 18 cm×22 cm and was on average100 μm thick.

The films prepared as above were placed in pairs in the bags and eithersmoothed manually before sealing or were sealed under vacuum followed byaddition of a known volume of air or nitrogen. The bags were made eitherfrom metallised polyester laminated to polyethylene or were bag-in-boxliners which contained an inner duplex liner of polyethylene as well asa sealed value socket. The area of each side of all bags was 18 cm×22cm.

The volume of air initially in each bag was between 200 ml and 300 ml.It can be seen that the film consumed oxygen highly efficiently.

EXAMPLE 23

Use of gamma-irradiation for activation of ethylene vinyl acetate (EVA)films.

The irradiation treatments and the bags were the same as in Example 22.The EVA films were cast from toluene solutions containing thecompositions shown below. The EVA was obtained as a gel (Morton ChemicalCo., USA under the trade name Adcote 1133.

The Cetyl alcohol was used as the photoreducing agent supplying labilehydrogen or electrons, a function which appears to be served by thepolymer itself in the case of the ethyl cellulose, cellulose acetate,and polyurethane adhesive (Lamal).

Cetyl alcohol, 0.32 g, triphenyl phosphite, 0.68 g, and2-ethylanthraquinone, 0.4 g, were dissolved in the toluene gel of EVA,12.5 g, to give a mobile solution. This was then cast into a film layeron the heat seal side of a 2 sheets of oxygen barrier plastic ofionomer/EVOH/polyester of oxygen transmission rate 1 cm³ /m² /24hr/atmosphere at 25° C., 75% RH. The area of film and thickness were asin Example 22.

The scavenging of oxygen present either prior to irradiation or injectedinto the bag after irradiation is shown in Table 6.

                  TABLE 5    ______________________________________             Oxygen %    Bag        Initial Final.sup.1                                Activatable Component    ______________________________________    PET/Pe     20.6    9.0      2-EtAQ.sup.2    PET/Pe     20.6    8.3      2-EtAQ    Bag-in-Box 20.6    2.8      2-EtAQ    Bag-in-Box 20.6    5.6      2-EtAQ    ______________________________________     .sup.1 3 days after irradiation     .sup.2 2ethylanthraquinone

                  TABLE 6    ______________________________________             Oxygen %    Bag        Initial Final.sup.1                                Activatable Component    ______________________________________    Bag-in-Box 20.6.sup.3                       14.1     2-EtAQ.sup.2    Bag-in-Box 20.6.sup.3                       10.8     2-EtAQ.sup.2    ______________________________________     .sup.1,2 as per Table 5     .sup.3 air injected 24 hours after irradiation.

EXAMPLE 24

Activation with Electron Beam

    ______________________________________    Films Cast    Sample    EtAq(g) TPP        TIP  EthCell(g)    ______________________________________    A         0.06    0.12            1.2    B         0.06               0.14 1.2    ______________________________________

Pouches were prepared in a similar manner to previous examples. Thetotal volume of the bags was measured, and this value was used tocalculate the volume of oxygen (from air) initially present. Oxygenanalysis was carried out 23 hours after the pouches were made, and thevolume of oxygen scavenged calculated. This was then converted into thepercentage of the stoichiometric amount scavenged (% Stoich.), whichcompares the actual volume of oxygen scavenged with the theoreticalmaximum which could be scavenged (assuming a 1:1 interaction of oxygenwith anthraquinone).

    ______________________________________    Results    Sample      Dose Rate (MRads)                             % of Stoich    ______________________________________    A           3.0          68    B           1.1          32    C           3.0          80    ______________________________________

The results show that the electron beam is an efficient method ofinducing photoreduction and bringing about oxygen scavenging, even atlower dose rates.

EXAMPLE 25.

Oxygen scavenging from a tinplate can

A crosslinkable polyurethane resin was used to demonstrate the use of anoxygen-scavenging coating on the inside of a tinplate can.

A can of volume 465 ml was coated internally with a solution of LamalHSA, 3.64 g, and Larmal C, 0.54 g, 2-ethylanthraquinone, 0.33 g, andtriphenylphosphite, 0.33 g, in ethanol 6 g. The solvents were evaporatedat 50-60° C. leaving a coating of polyurethane resin containing2-ethylanthraquinone and triphenylphosphite on the inside of the can.The can was then exposed to irradiation in the solar simulator for fiveminutes and then filled loosely with glass beads to reduce the headspaceto 170 ml and the can was then sealed by double-seaming.

The headspace gas was anaylsed after 24 hours by which time the oxygenconcentration had been reduced from 20.6% to 19.5%. The oxygen consumedwas 2.5 ml. This represents the quantity of oxygen which can be foundsometimes in commercial cans.

EXAMPLE 26

Oxygen Scavenging with a Copolymerised Reducible Organic Compound

Copolymers of 2-vinylanthraquinone were made with styrene (STY) and with2-hydroxyethyl methacrylate (HEMA). The copolymers containedapproximately 9 moles % of polymerised anthraquinonoid monomer.

Films were cast on the Polyester/EVOH/Surlyn barrier film as describedin other examples using the quantities shown below. The HEMA copolymerand its blend from ethyl cellulose were cast from ethanol and thestyrene copolymer was cast from a mixture of chloroform (70%) andacetone (30%). The films were made into pouches and the air was removedas in other examples. The pouches were irradiated for 5 minutes on eachside in the Suntester solar simulator and 20 ml of air was injected intoeach pouch, except for that containing the blend which was injected with50 ml of a mixture of 2.1% oxygen in nitrogen.

    ______________________________________    Quantities of Ingredients (g)          HEMA      STY              Cetyl Ethyl    Test  Copolymer Copolymer C.sub.6 H.sub.5 O).sub.3 P                                     Alcohol                                           Cellulose    ______________________________________    A     0.6       --        0.11   --    --    B     0.25      --        0.10   --    1.0    C     --        0.5       0.11   0.09  --    ______________________________________

    ______________________________________    Results              % Oxygen    Time (h)  Test A       Test B  Test C    ______________________________________    0         21.0         2.1     21.0    17.0      --           1.2     --    64.7      15.4         --      16.0    ______________________________________

The results show that the polymers scavenge oxygen, but theirpermeability to oxygen can result in slower scavenging than with ahighly permeable film such as ethyl cellulose.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

I claim:
 1. A method of lowering the concentration of ground statemolecular oxygen present in an atmosphere or liquid, said methodcomprising the steps of:(i) irradiating a solid phase compositioncomprising a source of labile hydrogen or electrons and at least oneorganic polymer, copolymer or oligomer bearing a reduciblesubstituent(s) selected from the group consisting of substituted orunsubstituted benzoquinone, 9,10-anthraquinone, and 1,4-naphthoquinone,with visible or ultraviolet light of a predetermined intensity orwavelength, γ-rays, corona discharge or an electron beam so as to reducethe reducible substituent(s) to a stable reduced substituent(s)oxidizable by ground state molecular oxygen; and (ii) exposing theatmosphere or liquid in substantial darkness to said irradiated solidphase composition such that the amount of ground state molecular oxygenpresent in the atmosphere or liquid is lowered through oxidation of thereduced stable substituent(s); wherein the oxidation of the reducedstable form of the reducible substituent(s) occurs regardless of thepresence of a transition metal catalyst and wherein the lowering of theamount of ground state molecular oxygen continues over a period of atleast 1 hour when conducted at room temperature.
 2. A method accordingto claim 1, wherein the lowering of the amount of ground state molecularoxygen continues over a period of at least 4 hours when conducted atroom temperature.
 3. A method according to claim 1, wherein the sourceof labile hydrogen or electrons is provided by the said polymer,copolymer or oligomer.
 4. A method according to claim 1, wherein thesource of labile hydrogen or electrons is provided by another polymer,copolymer or oligomer.
 5. A method according to claim 1, wherein thesolid phase composition is a polymeric film or polymeric film layer. 6.A method according to claim 1, wherein the solid phase compositionfurther comprises a scavenging component reactive towards activatedoxygen species resulting from the said oxidation of the reduced stableform of the reducible substituent(s).
 7. A method according to claim 6,wherein the scavenging component is a substituent(s) present on the saidpolymer, copolymer or oligomer.
 8. A method according to claim 6,wherein the scavenge component is a substituent(s) present on anotherpolymer, copolymer or oligomer.